1. Technical Field of the Invention
The present invention relates to an image forming apparatus for electrophotographically forming images such as copying machines, laser beam printers and the like, and more particularly to an image forming apparatus having a plurality of developing units.
2. Brief Description of Related Art
An image forming apparatus as mentioned above, has recently been designed to produce an image having different colors partially or an image consisting of various colors by providing a plurality of developing units to be interchangeably used in a round of image forming operations or by using a plurality of developing units interchangeably each time an image is formed.
However, such image forming apparatus is not provided with sufficient extra space since various equipments are installed inside the apparatus for the image forming operation, sheet transport, image fixing process, etc. Accordingly, the ability to equip a plurality of developing units around a photoconductor is limited. Especially for copying machines intended to be used for personal use, it has been is attempted to provide developing units of smaller size rather than enlarging the size of the body of the apparatus thereby necessitating a toner replenishing device that is also smaller in its toner accommodating capacity.
Among a plurality of developing units, a developing unit which accommodates black toner is used more frequently than the other developing units. Under such circumstance, if the capacity of a toner replenishing device becomes smaller, the toner accommodated therein is consumed very quickly, and therefore, frequent toner replenishment or frequent exchange of the toner replenishing device becomes inconveniently necessary for users.